which is faster ?

It is a very, very small difference. With the GUI you have to run the X Server, as a layer on top, to get the graphics.
Depends on what you are doing. If you know all the commands, you can fly through them without moving your hands from the keyboard. If you don't, you'll spend more time looking them up than rooting around in the GUI.

I use a combination of the two, and what I use depends on how much friction I expect each to provide.
command line or GUI ?
why ?
I have to agree with @GrumpyOldMan
Depends on what you are doing.
There are some things I want to do in a GUI such as a browser or LibreOffice, but for many things I do on a daily basis, the CLI, or "Command Line Interface", usually Bash, is the place to be! The CLI in Linux is in many instances, more powerful than the GUI, and sometimes the GUI is simply a wrapper for CLI commands!

Now for your initial question in this thread, question, "Which is faster, command line or GUI, and why?"

Say you have a 3 letter command that has both a CLI, and a GUI version, that produce the same results, with no user input, except to start and close the app. In the time I look up the command in the menu, or find the icon on the desktop, start the app, have my GUI bring up a window on the screen then execute the code within the app, then click to close the app, I can run the CLI version, get the result and have moved on to the next task!

Now combine that by chaining, and piping many different tools together on the CLI, into one command, otherwise known as "One-Liners", tools that might require you to execute several different GUI apps, copy and paste between the GUI apps to achieve the same results, and you see how powerful the CLI is over the GUI in many daily tasks.

For example, from: http://www.bashoneliners.com/
Show the largest 10 currently open files, the size of those files in Megabytes, and the name of the process holding the file open

 $ lsof / | awk '{ if($7 > 1048576) print $7/1048576 "MB" " " $9 " " $1 }' | sort -n -u | tail

Bottom line, use both as appropriate! Don't be afraid of the CLI! Embrace the power!

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